More capacity enlargement for a core transmission system is required along with the increase of the internet traffic. Under such condition, the coherent optical transmission technology is noted. An optical transmitter used in a coherent optical transmission phase-modulates an optical signal. And an optical receiver used in a coherent optical transmission receives a phase-modulated optical signal, mixes the received optical signal with the local oscillation light, generates a beat signal and reproduces transmission data from the beat signal.
In the coherent optical transmission technology, in particular, DP-QPSK can improve the wavelength utilization efficiency to double by performing the QPSK modulation for the orthogonal polarized optical waves with different signals respectively. For this reason, consideration for realizing a large capacity optical communication system using DP-QPSK is performed. Incidentally, DP-QPSK is an abbreviation of dual polarization-quadrature phase shift keying (polarization multiplexing with four phase shift modulation).
In recent years, it has begun to be studied to apply the coherent optical transmission technology to an inter-continental submarine optical transmission system. The yield strength to the deterioration of OSNR (optical signal to noise ratio) caused by a multiple cascade connection of the transponders is requested for the signal used in a submarine optical transmission system. For this reason, in a submarine optical transmission system, an RZ (return to zero) code in addition to an NRZ (non return to zero) code used widely in a land optical transmission system is also used. And by applying the coherent transmission technology to an RZ code transmission system, it is expected to improve the transmitter OSNR.
In a coherent optical transmission system using an RZ code, the light which a CW (continuous wave) light source outputs is inputted to an RZ modulator and an RZ pulse train is generated. The RZ modulator which generates an RZ pulse train from the CW light is called the RZ carver. The RZ pulse train generated at the RZ carver is inputted to the DP-QPSK modulator. The RZ pulse train inputted to the DP-QPSK modulator is separated into two, and QPSK modulation is performed by the respective different data. QPSK-modulated two RZ pulse trains are multiplexed so that the polarizations of the modulated light become orthogonal each other and transmitted to a transmission line.
In relation to the present invention, PTL (patent document) 1 discloses a phase modulation communication system which branches light signals amplitude-modulated by a clock, and performs the polarization mixing after the phase modulation for the respective light signals by the different data. PTL 2 discloses an optical transmission apparatus with which an optical phase modulator and an optical intensity modulator have been connected in series.